Tailings and Fine-Grained Waste Material

In general, tailings are the waste material from ore that has been processed to recover a concentrated form of the target commodity. Processing usually includes some form of grinding or milling of the ore, and as a result, tailings are usually fine grained (<2 mm). For most tailings, material on the surface that is >2 mm may be the result of a cover or extraneous material deposition that should not be included in the composite sample. Sieving to <2 mm ensures that the sample collected is primarily the tailings material. Although unlikely in tailings piles, if most of the tailings have a grain size >2 mm, use the methodology described in the “Material Containing a Mix of Small and Large Particles” section or the “Large Material Predominantly Greater Than 30 Centimeters” section for larger material types. One composite sample will be collected for each sample unit, not including duplicate samples or additional grab samples.

Material Containing a Mix of Small and Large Particles

It is not uncommon to have mine waste that is a mixture of fine material (<2 mm) and larger pieces (between 2 mm and 30 cm), such as in an ore stockpile that has had some initial bulk crushing but was never milled. In these piles, the <2 mm fraction and the >2 mm fraction are important to collect and analyze. Site history will help determine whether the >2 mm fraction is important to collect. Two composite samples will be collected at each sample unit, not including duplicates (described in the “Duplicates and Blanks for All Solid Sample Types” section) and additional grab samples. The approach consists of sieving the material to <2 mm, retaining the >2 mm fraction, and additional processing for larger pieces of material, as needed, to prevent a single large piece from biasing the composition of the >2 mm fraction. The two fractions will provide end-member compositions for the overall waste pile.

Large Material Predominantly Greater Than 30 Centimeters

Some waste material, such as waste rock, may have very little fine material and only large pieces that can range from several kilograms to many hundreds of kilograms. For waste material with large pieces, a <2 mm fraction will not be collected. The following sections present a method to subsample large pieces to collect a representative composite sample.

Drill Cores

Drilling is not required for site characterization, but if equipment and budget allow, it is encouraged because samples can be collected at depth in the pile. The details of the best equipment, techniques, and PPE for drilling at the site will be at the discretion of the State agency conducting the work. Tailings and other fine-grained mine waste are good candidate materials for drilling, resulting in a poorly consolidated core with some larger clasts possibly present. The methods described in the “Collecting Composite Samples” section assume the core is relatively unconsolidated and does not need a rock saw to split or collect samples. If the core does not meet this description, a discussion with USGS Earth MRI personnel may be warranted.

Personal Protective Equipment

Bottle Sets

Each sample collected will be distributed (“split”) into a set of the following 5 bottles:

Label bottles with waterproof labels or laboratory tape. If labeling bottles with tape, wrap the tape around the entire circumference of the bottle and secure by overlapping to prevent detachment. Waterproof labels can be printed or labeled with permanent ink. Uniquely label each bottle with the site name, date and time of collection, the analysis, and associated preservation. It is helpful if the label also indicates the program (Earth MRI–Mine Waste Characterization) and State. Add abbreviations for preservation to the sample name according to table 1; for example, label unfiltered metals and cation bottles with the suffix “-RA” (raw, acidified), filtered metals and cations bottles and precious metals bottles with the suffix “-FA” (filtered, acidified), filtered anions bottles with the suffix “-FU” (filtered, unacidified), and filtered alkalinity bottles with the suffix “-FALK” (filtered, alkalinity).

In the first example label in figure 3A, the “_001” is added if multiple samples are collected from the same site on different days (time series); the second sample could be indexed to “_002.” In the second example label in figure 3B, the time and date are incorporated into the unique ID; however, take care to prevent typographical errors in long strings of numbers, such as a date, when writing the labels. The abbreviation for a specific location (in this hypothetical example, Big Pit Lake [BPL]) may depend on where and how many samples are collected. If sampling multiple locations at a single large feature, “BPL01” may be used to delineate specific locations. In all examples, ensure details about the site, sample location, date, time, and so forth, are clearly described in the field sheet notes.

Acid Washing Bottles for Cation and Metal Sample Splits

Acid wash all metals, cations, and precious metals bottles before use. Some laboratory supply companies have the option to purchase precleaned bottles, but precleaned bottles are not required as long as the bottles can be sufficiently cleaned. Also acid wash a water collection container before using. If acid washing bottles, ensure the acid bath is a 10–20 percent reagent grade nitric or hydrochloric acid solution, and the bottles and caps are soaked for at least 12 hours. Rinse the bottles well and at least three times with DI water (for example, 18 megohm-centimeter or similar). Air dry the bottles in a clean environment (for example, in a laminar flow hood). Make a fresh acid bath every 6 months.

Deionized Water Rinse for Anion and Alkalinity or Acidity Sample Bottles

Soak anion and alkalinity bottles and caps overnight in DI water before use. Some laboratory supply companies have the option to purchase precleaned bottles, but precleaned bottles are not required as long as the bottles can be sufficiently cleaned. Do not acid wash bottles for anion and alkalinity sample splits. Acid washing anion and alkalinity bottles may result in erroneous chloride and alkalinity measurements.

Cleaning Pumps and Tubing

Peristaltic pumps are generally the best option for collecting water samples, as long as the height of the pump above the waterbody does not exceed the hydraulic head rating of the pump. If the water needs to be pumped to a height that exceeds the pump rating, then an alternate pump will need to be considered. Acquire tubing that is compatible with the pump and pump head used, rinse the tubing with DI water, and air dry completely before sampling. If feasible, have tubing dedicated for each site to prevent carryover across sites. However, if the tubing needs to be reused, rinse the tubing with DI water in the field, if possible, or thoroughly rinse with sample water before collecting the sample. Do not rinse with sample water if sampling water with low concentrations immediately following water with high concentrations, or if the relative concentrations cannot be determined in the field based on geochemical context.

Filters

Preferred filters are clean, unused 0.45 micrometers (µm) pore size, high-capacity capsule filters, although other 0.45 µm filtration systems are acceptable. Examples of other filters include plate filters, syringe and syringe filters. Prepare the filters according to the manufacturer’s instructions, if applicable. Record the filtration type and membrane material on the field sheet.

Water Collection Containers

Depending on the site, extra containers to collect raw samples for filtration or splitting into bottles may be required. Containers may be needed if water cannot be directly pumped from the source. The container(s) may be a carboy, a large bottle, or other clean (acid-washed) container that can hold enough sample to rinse equipment, bottles (as needed), and collect sample splits. The volume needed for sample collection will depend upon the type of equipment being used; for example, if a capsule filter is being used, use a minimum of a 2.5–3-liter (L) water collection container so that the filter can be rinsed with at least 1 L of sample before sample splits are collected per the manufacturer’s instructions. Rinse the water collection container(s) three times with the sample before filling. Rinse and fill a separate container for field parameters at the same time, if in situ (electrodes placed directly into the source water) measurements are not possible.

Additionally, access to water may require a collection mechanism such as a dipper sampler (for example, a “bottle on a stick”) which allows for sampling water from a distance as much as 2–3 m. This device is useful at adits, ponds, and pit lakes where there may not be direct, easy access (for example, a water tap). There are commercially available dipper samplers, but homemade samplers are also acceptable. Clean (acid wash) and change the dipper container between sites. Reconnaissance and (or) discussion with personnel familiar with the site may be helpful to determine if a dipper or other sampling devices are necessary.

Preservation Reagents and Distribution Equipment

Water sample splits have specific preservation requirements, illustrated in figure 2 and described in detail in this section.

Preservation for samples with the suffix “-FU” and “-FALK”.—Store the samples in a cooler with ice in the field and during shipping to USGS Sample Control. The samples may be stored on ice or refrigerated in the laboratory after collection and before shipping.

Field preservation for samples with the suffix “-FA” or “-RA”.—Samples with the “-FA” or “-RA” suffix need acidification in the field to preserve the sample for analysis. Use concentrated (16 molar) high-purity nitric acid (HNO₃) for preservation. The acid can be added to the sample after collection, or prewashed bottles with nitric acid preservative added by the bottle manufacturer can be used. If adding the nitric acid after the sample is collected, use a distribution system that includes either (1) a pipette, clean pipette tips, and a bottle for acid (Teflon preferred), or (2) a dripper bottle calibrated in the laboratory for how many drops approximately equals 1 mL of acid.

Water for Blanks

Bring DI water into the field for a field blank. The volume of water will depend on the length of tubing and the filtration system used; ensure the volume is sufficient to replicate the entire water collection process with DI water, including rinses and filling a complete bottle set.

Extra Equipment for Water Sampling

Even with the best planning possible, it is almost always necessary to have extra supplies. Consider packing at least 10 percent extra bottle sets, filters, preservation materials (if not using prepreserved bottles), DI water, water collection containers, and other supplies. If the water has a lot of particulates, additional filters may be needed. For example, if using 33-mm diameter syringe filters, having 5–10 filters per sample may be needed. One high-capacity capsule filter is usually sufficient per site, even with high particulate loads.

Equipment Reuse

Some equipment used for sampling cannot be reused. Bottles used in bottle sets are single use and will not be returned. Similarly, all filters are single use and not for use at multiple sites, across duplicate samples, or on multiple dates. If using a pipette to distribute acid into bottles requiring preservation, a single tip can be used to distribute acid into multiple bottles at a single site as long as the tip does not touch the sample water or a dirty surface. Avoid reusing pipette tips across sites.

Some equipment may be reused if cleaned between sites. Items include pump tubing and collection containers. If possible, tubing and collection containers dedicated to a site are preferable, but thorough DI water and sample rinses between sites may be used if necessary. Bring extra DI water into the field if rinsing equipment between sites.

Field Parameters

If possible, it is best to measure field parameters in situ. If in situ measurements are not feasible, a flow-through cell, bottle, or flask may be used. If measuring in situ, place the probes downstream from where the water sample is being collected or collect water samples immediately before or after measuring field parameters to prevent contamination. A meter or meters will be needed to measure the output from each electrode and perform the calibrations. If carefully selected, the same meter may be used for field parameters such as pH, T, SC, ORP, and DO (if a probe is used). Alternatively, a sonde with the ability to read the output and perform calibrations may be used. Consider testing electrodes and meters in the office or laboratory before conducting field work to ensure performance and familiarity with the equipment. It is not uncommon for mine waters to degrade electrode performance; therefore, consider bringing spare electrodes to the field. Consider measuring ORP and DO (optional parameters) if the equipment is available. The following are specific equipment and supplies needed to conduct the various measurements:

Rinsing with Sample

Rinse the tubing with sample by pumping water through the tubing without collecting a sample; the amount of water may depend on the length of the tubing used but should be approximately 1 L. Similarly, rinse the filter accord to the manufacturer’s instructions (for example, at least 1 L for capsule filters). Read the manufacturer’s guidance for specific filter requirements. The tubing and filter can be rinsed simultaneously. Rinse the bottles three times with sample water before collecting a sample, unless the bottles are prefilled with preservative. If a bottle is prefilled with preservative, fill it with the sample directly without rinsing.

Filtration

For the raw metals and cations sample, do not filter when collecting the sample. Ensure all other samples go through a 0.45 µm filter before collection. Fill the bottles to the shoulder of the bottle (~100 mL), except alkalinity samples—fill those bottles to the top and leave minimal headspace in the bottle.

Preservation of Water Samples

For raw and filtered metals and precious metals samples, acidify the sample in the field to 1 percent volume per volume nitric acid. For example, if a 125 mL bottle is filled to the shoulder and contains approximately 100 mL of sample, add 1 mL of concentrated nitric acid to the bottle. Wear safety goggles and gloves while acidifying, and shake the bottle after capping to mix. For anions and alkalinity samples, store filled bottles in a cooler on ice in the field with no additional preservation. Keep ice and samples in separate sealable plastic bags to prevent melted ice from contaminating the samples.

Quality Assurance and Quality Control—Duplicates and Blanks

For each sampling event (date), collect at least one DI water field blank to test the cleanliness of the sampling process. Follow the entire sample collection protocol with DI water, including rinses and a complete set of sample bottles. Collect a duplicate sample set at least once per sampling trip or 1 per every 20 sample sites, if more than 20 sample sites are collected in a single sampling event.

Sample Handling, Storage, and Shipping

Clearly label each bottle as described in the “Equipment and Preparation” section. Group samples of a single analytical type (for example, alkalinity or precious metals) into a sealable bag; bags of samples with mixed analytical types are not acceptable. Multiple samples may be stored in a single plastic bag as long as they are of a single analytical type, but keep the bag sealed and consider double bagging the samples. Store samples with caps tightly closed to prevent leakage and in sealed plastic bags to prevent melted ice or dirt from contaminating the samples.

Keep anion and alkalinity samples chilled in the field, during shipment, and when stored in the laboratory. Metals and precious metals samples may be chilled, but it is not required. Do not freeze samples at any time. Inventory samples before shipping them and submit them to USGS Sample Control (refer to the “Sample Submission and Geochemical Analysis” section).

pH

Measure pH in the field, either in situ or in a flask or beaker filled with a sample as close to ambient conditions as possible. Because pH is T dependent, it is important to measure T simultaneously with pH and to calibrate at ambient T for automatic T compensation. The following steps are for calibrating and measuring pH in samples:

If the pH of the water falls outside of the buffers used for calibration, note this condition on the field sheets and in the final data report.

It is preferable to measure pH in the field. However, if it is not possible, an additional sample split may be collected in a clean bottle by filling the sample to the top of the bottle and chilling the bottle during transport and storage at the laboratory. Ensure measurements are made within 48 hours and at room temperature to minimize changes in water chemistry. Note if there are visible precipitates or color changes in the sample split. Also note this deviation from field measurement on the field sheet.

Specific Conductance

Because SC is highly T dependent, it is important to measure T simultaneously with SC for automatic T compensation. The following steps are for calibrating and measuring SC in water samples:

It is preferable to measure SC in the field. However, if it is not possible, an additional sample split may be collected in a clean bottle by filling the sample to the top of the bottle and chilling the bottle during transport and storage at the laboratory. Ensure measurements are made within 48 hours and at room temperature to minimize changes in water chemistry. Note if there are visible precipitates or color changes in the sample split. Also note this deviation from the field measurement on the field sheet.

Temperature

Temperature can be measured using a separate thermistor or thermometer, or more commonly, using the thermistor incorporated into the SC probe or a pH triode. If measured with an SC probe or pH triode, record which probe or electrode’s T value is being used on the field sheet and use that same source consistently through the sampling program. The following steps are for calibrating and measuring T in samples:

Oxidation-Reduction Potential

An optional measurement is ORP, which measures the potential in a water sample across a platinum electrode, relative to the standard hydrogen electrode. ORP measurements are susceptible to several limitations, including disequilibrium conditions in the water and slow or low electrochemical response to the electrode surface for some redox couples. However, in waters with high iron concentrations, such as some mine-affected waters, ORP can be a valuable qualitative measurement.

Measure ORP in the field, either in situ or in a flask or beaker filled with sample as close to ambient conditions as possible. The following steps are for calibrating and measuring ORP in samples:

It is preferable to measure ORP in the field. However, if it is not possible, an additional sample split may be collected, stored in a clean bottle, filled to the top of the bottle with sample water, and chilled during transport and storage at the laboratory. Ensure measurements are made within 48 hours to minimize changes to sample chemistry, and do not proceed if there are visible precipitates or color changes in the sample split. Record this deviation in the field measurement on the field sheet.

Dissolved Oxygen

Another optional measurement is DO, which can be measured by a colorimetric method (Rhodazine D or Indigo Carmine) in the field with a commercially available kit or a DO probe and meter. The kits rely on reagent ampules that react with sample water to produce a color. The kits may have a calibration color chart or use a portable spectrophotometer to convert color saturation in the reacted ampule to a concentration of DO. It is important that the manufacturer’s instructions are followed carefully, particularly regarding collection requirements and wait times for analysis because inclusion of atmospheric oxygen can compromise the analysis. There are several caveats with this technique that may be encountered at mine or mill sites:

Dissolved oxygen may also be measured by a meter and calibrated DO probe, if available. There are several types of probes that each have unique maintenance requirements. A meter and probe setup is more expensive than the colorimetric method and requires calibration by one of several methods. In addition, membrane probes may become clogged by ferric iron precipitates in mine-affected waters and produce incorrect results.

Regardless of which method and manufacturer is chosen for DO measurements, record the details on the field sheets. It is important to measure DO in situ or as close to the source water as possible because DO will ingas or degas throughout time and as the water T changes. Also, relative saturation depends upon T and elevation, and it is important that these parameters are recorded on the field sheet.

Metals and Other Elemental Analyses

Cations and metals are measured by preparing the sample using a sodium peroxide fusion method in which samples are fused at 750 degrees Celsius (°C) with sodium peroxide and the fusion cake is redissolved in dilute nitric acid. Sixty-one elements are analyzed in the dissolved fusion cake by inductively coupled plasma mass spectrometry (ICP–MS) and ICP optical emission spectroscopy (ICP–OES). Elemental analytes are Ag, Al, As, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Ho, In, K, La, Li, Lu, Mg, Mn, Mo, Nb, Nd, Ni, P, Pb, Pr, Rb, Re, S, Sb, Sc, Se, Si, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, Tm, U, V, W, Y, Yb, Zn, and Zr. Major elements are determined by dissolving the sample into a lithium metaborate and tetraborate fusion disk and measuring major element oxides by wavelength dispersive X-ray fluorescence spectrometry. Analytes are Al₂O₃, BaO, CaO, Cr₂O₃, Fe₂O₃, K₂O, MgO, MnO, Na₂O, P₂O₅, SiO₂, TiO₂, SrO, V₂O₅. Loss on ignition is also measured. Gold, palladium, and platinum are determined by a lead fusion fire assay in which the precious metals are separated from the rest of the sample in a flux, followed by lead and other base metals removal. The precious metals are dissolved in aqua regia and analyzed by ICP–MS and ICP–OES. Inorganic carbon is determined by treating the sample with perchloric acid (HClO₄) and measuring the resulting carbon dioxide with an infrared detector. Total sulfur and total carbon are determined by sample combustion and infrared detection of sulfur dioxide and carbon dioxide gas. Mercury is determined by cold vapor atomic absorption spectroscopy. Fluorine is determined by digestion and measurement with an ion-selective electrode.

Mineralogy

Mineralogy is determined by powder X-ray diffractometry using the spiked Rietveld method (Albinati and Willis, 2006) by USGS laboratories.

Acid-Base Accounting

The method used for acid-base accounting is based on the method developed by the U.S. Environmental Protection Agency (Sobek and others, 1978). A sample aliquot is subjected to a preliminary fizz test to determine the volume and concentration of acid needed for the analysis. Another aliquot is reacted with water to measure the paste pH. Based on this information, the sample is dosed with acid and backtitrated with a base. From this information, the neutralization potential (NP), the acid generating potential (AP), and the net neutralization potential (NNP) can be calculated.

Quality Assurance and Quality Control

The USGS QA/QC practices include the submission of analytical duplicates and standard reference materials as unknowns to the analytical laboratories for all methods. Twenty percent of the submitted samples are analyzed as analytical duplicates. In addition, reference materials similar to the submitted matrix are included as blind samples to the laboratory. The performance of the duplicates and reference material analysis is evaluated and reported to the submitter.

Quality Assurance and Quality Control

In addition to field blanks and duplicates, the USGS will submit additional reference water samples as blind samples to the laboratory. The performance of the blanks, duplicates, and reference material analysis is evaluated and reported to the submitter.